This invention relates to a process for plugging high permeability earth strata. In particular, it relates to a process for plugging high permeability earth strata using foam containing condensible gas and a surfactant.
To increase oil well production, steam can be cyclically injected into the strata to reduce the viscosity of the oil in the vicinity of the well bore, so that oil flows more readily into the well bore (this method when repeated is called cyclic steam stimulation or the "huff-puff" system). The production can also be increased by injecting steam into a well bore for the purpose of pushing oil from the oil-bearing formation into another nearby well (this is called a direct steam drive system). If the formation around the well bore includes strata of varying permeability, to allow stimulation of production by either technique from less permeable strata, the highly permeable strata are plugged prior to steam injection. One of the most successful methods of plugging the permeable strata is described in U.S. Pat. No. 3,412,793. In accordance with this method, a surface-active agent is added to the steam and caused to foam by action of the steam pushing through the surfaces of the more permeable strata. The foam has steam as its gaseous phase and is, therefore, stable only as long as the temperature remains above the boiling point of water at reservoir pressure. When the temperature drops below that point, the foam collapses, thereby restoring the original permeability to the previously plugged strata and allowing oil to flow therethrough. In some applications it is desirable to plug only a portion of the strata.
One problem associated with plugging strata or portions thereof using steam-surfactant foam stems from the fact that the strata to be plugged is initially at a temperature lower than the boiling point of water (at operating pressures). Thus, steam-surfactant mixture introduced initially into the highly permeable strata serves merely to raise the temperature of strata and a stable foam does not form until the temperature, in the portion of the strata into which the mixture is injected, reaches the temperature above the boiling point of water at operating pressures. Not only is a portion of the steam-surfactant mixture wasted during the initial stage of operation, but also, large amounts of liquids formed inside the formation can interfere with subsequent oil production and accelerate the heat transfer from the foam, thereby reducing its useful life. Furthermore, to increase the heat transfer during the heating stage of the operation, the steam-surfactant mixture is at an extremely high temperature. This limits the choice of surfactants to those which can withstand those temperatures without undergoing thermal decomposition. Even when relatively high temperature steam is used, several days of steam injection are required to heat up the strata to a point where a stable foam can be formed.
The present invention obviates the problem discussed above.
Thus, one object of the present invention is to provide an improved process for plugging permeable strata or portions thereof using foam which contains steam as its gaseous phase.
Another object of the invention is to increase the efficiency of the process for plugging highly permeable strata or portions with foam formed with steam as a gaseous phase.
A further object of the invention is to reduce the amount of water formed in the strata during plugging operation.
Still another object of the invention is to reduce the amount of steam-surfactant mixture needed for plugging of a highly permeable strata or portions thereof.
A still further object of the invention is to reduce the energy requirements of the process for plugging permeable zones.
Still another object of the invention is to reduce the time required for plugging zones of a permeable strata.
A still further object of the invention is to increase the number of surfactants that can be chosen for the use in the process of plugging permeable strata.
Still another object of the invention is to provide an improved process for stimulation of production from oil wells.
Other objects of the invention will become apparent to one skilled in the art upon studying this disclosure.